A red box, for those of you not in the know is a device that allowed you to make free pay-phone calls. When a quarter was dropped into a pay-phone, five little tones were sent down the line to the central office telling them that a quarter was deposited (a dime is two tones, one tone equaled five cents). Through secret channels and text files the instructions were passed along.
It was so simple to build and use that it was almost a joke. Since I was in college I'd build red boxes for anyone who wanted to give me the $10 it cost to make one. I think if a hack like that were to come out now, it'd be routed around within a year.